Your original description is mostly clear but can be improved slightly for grammar and flow. Here is a corrected version:
"Dry grasslands are open, windy areas with few trees and little rain. They can even have sandstorms. In Eurasia, dry grasslands are called the Great Steppe. In America, people call them prairies."
Changes made:
- Added a comma between "open" and "windy" because when two adjectives equally describe a noun, they should be separated by a comma.
Otherwise, your original sentences are grammatically correct and easy to understand for a 7-year-old. Great job!